1. Field of the Invention
The present invention relates to apparatus and methods for analyzing body sounds. More particularly, the present invention relates to visual and computer assisted analysis of digitized body sounds.
2. Description of the Prior Art
The normal human heart is a four chambered structure, shown schematically in FIG. 1. It is arbitrarily divided into a right side (patient's right side) which accepts deoxygenated blood returning from the body through the venae cavae and pumps this blood into the lungs through the pulmonary artery. The lungs re-oxygenate the blood and excrete carbon dioxide. The re-oxygenated blood returns to the left side of the heart through the pulmonary veins and is pumped to the body through the aorta.
Both the right side and the left side of the heart have ventricles which actively pump blood through the contraction phase of the cardiac cycle (called systole) and atria which function to assist ventricular filling during the relaxation phase of the cardiac cycle (called diastole). On the right side of the heart, the tricuspid valve separates the right atrium and ventricle. The pulmonic valve separates the right ventricle and pulmonary artery. On the left side of the heart, the mitral valve separates the left atrium and ventricle, and the aortic valve separates the left ventricle and aorta.
Normal heart sounds are produced by the closure of the valves separating the atria from the ventricles (called the first heart sound, S1) and the subsequent closure of the valves separating the ventricles from their attached arteries (called the second heart sound, S2). The first heart sound, S1, has two components, T1 and M1. T1 is caused by the closure of the tricuspid valve, between the right atrium and right ventricle. M1 is caused by the closure of the mitral valve, between the left atrium and the left ventricle.
Similarly, the second heart sound, S2, has two components, A2 and P2. A2 is caused by closure of the aortic valve, between the left ventricle and aorta, and P2 is caused by closure of the pulmonic valve between the right ventricle and pulmonary artery. In a normal individual, mitral valve closure (M1) precedes tricuspid valve closure (T1) slightly. Aortic valve closure (A2) normally precedes pulmonic valve closure (P2) by a varying amount, depending upon the phase of the respiratory cycle. Normally, A2 precedes P2 by a longer period during inspiration than during expiration. All of these normal heart sounds are within the low frequency end of the human hearing range, falling between 30 and 250 Hz in frequency.
Abnormal heart sounds may be produced by the rapid filling of dilated ventricles, producing a third heart sound called S3, as well as by the contraction of the left atrium against a non-compliant left ventricle, producing a fourth heart sound called S4. Other abnormal heart sounds and murmurs may be produced by a variety of different pathological conditions.
The timing of abnormal hearts sounds relative to other heart sounds, to the respiratory cycle, and to the electrical impulses causing the heart to beat, is important in diagnosing the condition causing the abnormal sounds. FIG. 2 shows the relationship between the electrical impulse (normally detected by an electrocardiogram or ECG), the respiratory cycle, and normal heart sounds. Ventricular excitation is detected by the QRS complex of the ECG. When the ventricles are electrically excited, contraction occurs which results in increases in ventricular pressure. When the pressure in the left and right ventricles exceed that in their corresponding atria, closure of the mitral valve (M1) and the tricuspid valve (T1) occur, respectively. Usually, M1 and T1 overlap, so that S1 is one continuous sound rather than being split.
When ventricular contraction ceases and relaxation of the ventricular muscle occurs, pressure decreases in the ventricle. When the pressure in the left and the right ventricles falls below that of the aorta and pulmonary artery, respectively, aortic closure (A2) and pulmonic closure (P2) occur. The sum of A2 and P2 form the second heart sound (S2). S2 is usually split into separate, identifiable A2 and P2 sounds. The period between A2 and P2 is normally greater when the patient is inhaling than when the patient is exhaling.
Changes in the timing relationship or intensity of these normal sounds can indicate a physical problem. The existence of extra, abnormal heart sounds also frequently indicates some physical pathology. Various clues assist the physician in determining what condition is causing the extra sound. Frequency and pitch of extra sounds, their timing and duration, and their intensity are all related to their cause. Physiologic maneuvers, such as hand grip and valsalva (expiration against a closed glottis), which alter the amount of venous return as well as left ventricular after load, can be used to accentuate or diminish the intensity of some abnormal heart sounds and murmurs, and can, thus, be used to aid in differential analysis.
Normally, blood flow is not audible through peripheral arteries such as the carotid arteries (in the neck), the abdominal arteries (supplying the kidneys, intestines, etc.) and the extremities (e.g. femoral arteries). However, when these vessels become narrowed (stenosed) by pathological processes (e.g. atheromatous plaques) or flow is increased by the development of a shunt or fistula (e.g. hemodialysis access), then blood flow through these arteries may become audible. In general, the pitch or frequency of the sounds from such flow will correlate with the severity of the narrowing. Moreover, the sound of flow during diastole, when the arterial pressure is generally lower, will be a more specific indicator that a significant stenosis or narrowing is present.
Breath sounds may also provide considerable information about pulmonary pathology. In general, breath sounds caused by air rushing into and out of the lungs during respiration tend to be most coarse over the trachea and major bronchi and much finer and softer over the peripheral lung fields. Pathologic processes such as consolidation (as might occur with pneumonia) may increase the transmission of coarser upper airway sounds in the lung periphery, whereas other processes such as fluid gathering around the lung (pleural effusion) may decrease the sounds heard over an area. Increases in lung water, as might occur with heart failure as well as other pulmonary conditions, produces an abnormal crackling sound at end-inspiration called rales.
The relationship between abnormal heart sounds, peripheral vessel sounds, and breath sounds and underlying physical pathologies has long been appreciated by cardiologists. However, clinical auscultation (examination by listening to body sounds) is an extremely difficult skill to master. The heart sounds are low pitched and close together, and it is difficult for humans to separate sounds out or remember sounds accurately. Even when auscultation is performed expertly, the data derived from the examination is expressed semi-quantitatively at best in the form of a note in the patient's file. No record of the actual data is available for further analysis or comparison with data from prior or subsequent examinations, or between observers.
A variety of inventions have been developed to assist physicians and other care givers with auscultation. None of these devices has been successful, due to several disadvantages discussed below. It is known in the art to provide a slowed down audio signal of a heart beat. See, for example U.S. Pat. No. 4,528,689 by Katz. It is also known to use an electronic stethoscope to display heart sounds visually. See U.S. Pat. Nos. 5,213,108, 5,218,969, and 5,010,889 by Bredesen et al., 5,025,809 by Johnson et al., 4,765,321 by Mohri, 4,254,302 by Walsh, 4,594,731 by Lewkowiez and 4,362,164 by Little. Some of these references discuss computer assisted diagnosis based on the heart sounds. It is also known to take frequency domain (e.g. fast Fourier transform) data of the heart sounds in order to aid in diagnosis. See, for example, U.S. Pat. Nos. 4,792,145 by Eisenberg et al., 5,301.679 by Taylor, 5,002,060 by Nedivi, and 4,720,866 by Elias et al.
None of these inventions are useful in normal diagnostic situations, because they do not provide any effective means of separating background noise, emanating from within the body or external to the body, from the body sounds to be analyzed. In addition, these inventions do not provide quantitative timing comparisons between body sounds, respiratory cycle, and electrical impulse.